My Dyna ST70, with a steel chassis has been used as a test bed for all kinds of vacuum tube amplifier circuits, over 20 years.
The problem was with single ended air-gapped output transformers, a choke input B+, and No negative feedback.
Hum usually was between 500uV and 1.5mV. Much more that I will accept, but it allowed me to test the other performance of those circuits.
Even the push pull versions that used a choke input B+, and No negative feedback had 500uV or more hum.
Magnetic parts were properly oriented, but the steel chassis did cause hum to be a problem.
That was just one chassis.
Still, using No negative feedback:
I now have push pull, self Inverting push pull, and balanced amplifier circuits.
With all of my amplifiers that use an aluminum chassis, my hum is < 100 uV (Less than 100 Micro Volt).
If I remember correctly, the Dyna ST70 steel chassis with my push pull design, with No negative feedback, the hum is about 200uV.
Magnetic steel, not for me.
Just my experiences.
PS All of the amplifiers I designed that are using an aluminum chassis, are using . . . Heathkit chassis.
Who says all Heathkits used steel chassis?
Only the power transformer and aluminum remains. Everything else was removed. My "new" amplifier is completely different versus the original Heathkit circuit.
The problem was with single ended air-gapped output transformers, a choke input B+, and No negative feedback.
Hum usually was between 500uV and 1.5mV. Much more that I will accept, but it allowed me to test the other performance of those circuits.
Even the push pull versions that used a choke input B+, and No negative feedback had 500uV or more hum.
Magnetic parts were properly oriented, but the steel chassis did cause hum to be a problem.
That was just one chassis.
Still, using No negative feedback:
I now have push pull, self Inverting push pull, and balanced amplifier circuits.
With all of my amplifiers that use an aluminum chassis, my hum is < 100 uV (Less than 100 Micro Volt).
If I remember correctly, the Dyna ST70 steel chassis with my push pull design, with No negative feedback, the hum is about 200uV.
Magnetic steel, not for me.
Just my experiences.
PS All of the amplifiers I designed that are using an aluminum chassis, are using . . . Heathkit chassis.
Who says all Heathkits used steel chassis?
Only the power transformer and aluminum remains. Everything else was removed. My "new" amplifier is completely different versus the original Heathkit circuit.
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I was merely pointing out that 2mm steel is a pig to work with, will be next to impossible to bend easily, and could lead to a clunky solution. Generally 1mm is a lot easier to work with - for example using hole punchers.
I have also used steel chassis with no issue from hum. I generally build with toroids, with the power transformer underneath the chassis, so maybe that makes it easier.
It's possible (but not a barrel of laughs) to punch Bud / Hammond steel chassis with a Greenlee punch. Grease everything, especially the threads, and have at it. It's even possible to punch the little cast aluminum Hammond boxes, if you're willing to place one foot on the box and the other on a ratchet. (Punch still fine, ratchet ?).
The dome of the Washington Monument in DC is covered by the most exotic and expensive material of its day (just post American Civil War) - aluminum. Any kind of chassis weight bearing can be done with cheap welded aluminum chassis with internal struts of C-channel or even L-channel extrusions. Not classic, but easy enough for stuff not needing magnetic field shielding.
If you want to make a hole for an IEC jack, or maybe a cord D-shaped strain relief, and can't justify the cost of the punch, you have to decide whether or not you're a real metalworker. Do you like filing and precision metalwork? Aluminum is faster, steel is potentially prettier. Ginger or MaryAnn?
All good fortune,
Chris
The dome of the Washington Monument in DC is covered by the most exotic and expensive material of its day (just post American Civil War) - aluminum. Any kind of chassis weight bearing can be done with cheap welded aluminum chassis with internal struts of C-channel or even L-channel extrusions. Not classic, but easy enough for stuff not needing magnetic field shielding.
If you want to make a hole for an IEC jack, or maybe a cord D-shaped strain relief, and can't justify the cost of the punch, you have to decide whether or not you're a real metalworker. Do you like filing and precision metalwork? Aluminum is faster, steel is potentially prettier. Ginger or MaryAnn?
All good fortune,
Chris
t is not an issue for me to make holes in the chassis. I see that many people on this forum are concerned about it, but for me it is not an issue. All holes are pre-drilled or milled out. I'm a bit skeptical about the advice I've received regarding the steel chassis, and the problems with hum I might get... Thanks to you Chris. Maybe I give the steel chassie a chance?
Your schematic in Post # 1 . . .
With a single ended output transformer that has Es & Is separated by an air gap, might not have a hum problem because it has
Global Negative Feedback, which does cover a multitude of sins (and sometimes creates a few sins of its own).
A toroid output transformer is usually less affected by other parts magnetic field eminations.
Given the above, perhaps a magnetic steel chassis will be OK in the "hum department".
Next:
Watch out for ground loops that cause hum.
To get less than 100 uV of hum and noise, usually does require solving one or more causes of hum and noise.
With a single ended output transformer that has Es & Is separated by an air gap, might not have a hum problem because it has
Global Negative Feedback, which does cover a multitude of sins (and sometimes creates a few sins of its own).
A toroid output transformer is usually less affected by other parts magnetic field eminations.
Given the above, perhaps a magnetic steel chassis will be OK in the "hum department".
Next:
Watch out for ground loops that cause hum.
To get less than 100 uV of hum and noise, usually does require solving one or more causes of hum and noise.
It is OK to use steel. This is a small headphone amplifier, no global feedback, mounted on a laser drilled steel plate (1.6mm thickness). No audible hum. The transformers are fully shielded and properly oriented.
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I always start with my speakers and how loud I want to listen to my music. I like efficient speakers (As Paul Klipsch stated, distortion is inversely proportionally to efficiency). So, the speaker is the first element in the sound chain, not the last.
The more efficient the speakers, the more sensitive they are to residual hum in the amplifier but the less power is needed for a given sound level (SPL).
What speakers will you use?
The more efficient the speakers, the more sensitive they are to residual hum in the amplifier but the less power is needed for a given sound level (SPL).
What speakers will you use?